Unlock after selling

I sold my old iPhone 4 a year ago (via craigslist) The buyer emailed today asking for a copy of the receipt because he wants to unlock it and AT&T told him:

In order to process your unlock request, we must receive a faxed copy of your original receipt. Please fax us a copy of the receipt including your Request Number and IMEI to 1-214-782-6442 within 48 hours.

I'd like to help the guy but I don't know him. What is best course of action? I don't feel right sending him the receipt with my billing address on it. I suppose I could redact it. Or does AT&T need that. Alternatively maybe I could send the info direct to AT&T. Suggestions?

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